The present invention relates to spectrochemical sources and more particularly to glow discharge sources.
Glow discharge (GD) plasmas have been used as spectrochemical (i.e., optical emission) sources for well over 100 years, dating back to the early studies of atomic structure. The low pressure, low power plasmas are easily controlled and yield emission spectra that are principally atomic in nature. The combination of cathodic sputtering as a means of introducing atoms from bulk solids and the relatively simple optical spectra lead to the implementation of hollow cathode GD devices as line sources for atomic absorption spectrophotometry. The development of the Grimm-type glow discharge geometry lead to the use of glow discharge optical emission spectroscopy (GD-OES) as a tool for both bulk solid and depth resolved analysis of metals and alloys. The subsequent introduction of radio frequency (rf) powering schemes opened up the scope of application further to nonconductive materials and coatings.
One of the strongest features of standard glow discharge devices is the fact that they operate in inert environments and are thus free from atmospheric contaminants. While the cathodic sputtering event entails sufficient energy to release neutral atoms and molecules from solid matrices, the discharge's gas phase temperature is insufficient to cause desolvation of analytes introduced in water vapor, a phenomenon that is typical in atmospheric pressure flames and plasmas. As such, a good deal of effort has been devoted to developing strategies for getting liquid-originating analytes into the discharge environment.
The most common method for getting liquid-originating analytes into the discharge environment involves drying an aliquot of analyte-containing solution on an inert target that is subsequently introduced as the cathode of the GD source so that the dried residue can be sputtered from the cathode's surface. In this way, solvent vapors are excluded from the discharge volume, and the plasma operated much in its “normal” manner. While effective, this approach is laborious and not amenable to what would ideally be the analysis of flowing streams such as liquid chromatograph eluents. To address this shortcoming, transport-type liquid chromatography-mass spectrometry (LC-MS) interfaces such as the moving belt and the particle beam have been used to introduce dried analytes into the plasmas in a continuous fashion. Schroeder and Horlick have also attempted to introduce nebulized solutions directly into a hollow cathode emission source with some level of success.
Over 40 years ago, Couch and Brenner described a phenomenon by which a glow discharge plasma was sustained at atmospheric pressure between a tungsten anode and an electrolyte solution that acted as the cathode. Solutions containing copper and indium dopants produced optical emission spectra analogous to that obtained in flame emission sources. On the other hand, solutions containing other cationic species (Li, Na, S, and U) did not yield characteristic spectra. The Couch/Brenner device was actually a modified version of a system that originally was described by Gubkin and later reviewed by Hickling and Linacre and was employed for very high yield electrolysis of aqueous solutions of metal salts.
Cserfalvi and co-workers reinvestigated this phenomenon as a means of analyzing dissolved metals in electrolytic solutions, coining the term electrolyte-cathode discharge (ELCAD). In their original apparatus, the electrolyte-containing solution was disposed in a basin having two regions separated by a glass frit. A graphite rod that was electrically maintained at the cathodic potential of the discharge circuit was submerged in one of the regions of the basin. A central inlet tube passed vertically through the other region of the basin. The analyte-containing solution was continuously re-circulated at flow rates of 2 to 10 milliliters per minute (mL/min) through the central inlet tube so as to form a small stationary “waterfall” with a slope of about 60 degrees at the edge of the central inlet tube. A tungsten electrode (acting as the anode) was mounted one to five millimeters (mm) above this slope of the waterfall. The glow discharge formed in the space between the end of the anode and the slope of the waterfall. The glass frit separated the region of the basin containing the waterfall from the region containing the cathode rod in order to eliminate the evolution of H2 gas and possible explosion. Current-voltage (i-V) plots generated for that device supported the assumption that the devices did indeed operate in the so-called “abnormal” glow discharge regime. Both operating voltage and observed analyte emission responses were dependent on the pH of the test solutions, with the authors suggesting that solution conductivity, and more specifically hydronium ion concentration, being a key aspect of the physical operation of the devices. Detection limits for more or less bulk solutions of metal analytes produced detection limits of approximately 0.1 to 1 part per million (ppm), though for total analyte solution volumes of more than 10 milliliters (mL).
Subsequent studies on the ELCAD source by Mezei, Cserfalvi, and Jánossy, sought to elucidate the operating mechanism of the device. The authors used a variable pressure cell to study the role of gas-phase collision frequency on the operating characteristics. In most cases, increases in gas (atmosphere) pressure from 500 to 1200 millibar (mbar) yielded greater emission intensities, which the authors ascribed to increased three-body recombination of analyte ions sputtered from the solution surface (i.e., M++e+e - - - M*+e). Neutralized atoms in the analytes could then in turn be excited in the plasma region immediately above the surface of the solution. Based on the known field structure in the vicinity of the cathode electrode in a glow discharge, the actual release of a cationic species from the surface of the solution seems very unlikely. The authors subsequently calculated a gas-phase temperature above the cathode surface based on an assumption of the kinetic energy of ions colliding with the liquid surface. A gas-phase temperature of approximately 7000 degrees Kelvin was suggested.
Kim and co-workers have recently described an extension of the studies of Mezei et al. by the use of an ELCAD system wherein argon is introduced as the discharge gas in a pseudo-closed vessel system that was purged through a bubbler. In their design, a platinum wire anode was placed opposite the analyte “waterfall” with analyte flow rates of 5 to 10 mL/min. The Ar gas served to also reduce the possibility of explosion, and the high solution flow rates kept the sample solution from boiling. This group performed parametric studies of the sorts described above, finding as well that the current-voltage (i-V) characteristics of the plasma were representative of an abnormal glow discharge with dependencies on both the pH of the solution and the inter-electrode gap. Interestingly, the authors observed no emission from the Ar discharge gas species, though in the wavelength range investigated (400-500 nm) only Ar (II) species would be expected to be present. Here too, the authors proposed a mechanism whereby ions of the analyte metals were sputtered from the solution (cathode) surface, subsequently neutralized in the cathode dark space and then excited within the plasma, with the parametric dependencies indicating that some sort of sputtering threshold must be realized prior to analyte release. Analyte emission intensities were found to come to steady state conditions following one minute of introduction at flow rates of 10 mL/min. Once stabilized, analyte stabilities of approximately 2.5% relative standard deviation (RSD) were obtained. Limits of detection (LOD) were subsequently calculated to be in the 0.001 to 1 ppm range.
Kim et al extended their work to an open-air cell that connected the cathode to the water in the cell by a platinum wire. The liquid sample was introduced through a peristaltic pump and flowed over the cathode. It was found that the surface tension of the sample solution made it difficult to maintain continuous flow over the cathode at flow rates lower than 5 mL/min. It also was found that maintaining the flow rate above 5 mL/min helped prevent the sample from boiling. This work yielded similar operating characteristics to the prior work of Kim et al, though the analytical performance of the open-air cell was degraded both as to the time required to reach the steady state (2 to 3 mins.) and the precision (less than 8.7% RSD in time and 2.9% for repetitive wavelength scans) of the analysis. Improvements in LODs of as much as one order of magnitude were observed for some transition metal elements, with values of 0.01 to 0.03 ppm being typical.